1. Field of the Invention
The present invention is related to the field of variable speed wind turbines, and, more particularly, to a variable speed wind turbine comprising a doubly fed induction generator (DFIG), an exciter machine, an intermediate static converter not connected to the grid, power control and pitch regulation.
2. Description of the Prior Art
In the last few years, wind power generation has increased considerably worldwide. This growth is widely forecast to continue into the next decades, even as the industry and technology have arisen to a mature level in this field. As wind farms grow in size and the total base of installed wind capacity continues to increase, the importance of improving the quality of power output becomes a challenge of huge importance to wind developers and utility customers alike.
Electric power transmission is one process in the delivery of electricity to consumers. A power transmission system is often referred to as a “grid”. Transmission companies must meet the challenge of getting the maximum reliable capacity from each transmission line. However, due to system stability considerations, the actual capacity may be less than the physical limit of the line. Thus, good clean sources of electrical power are needed to improve system stability.
In most applications, wind turbines generate electric power and feed current into the electric grid. This may cause deviations of the local grid voltage, such as a change of the steady state voltage level, dynamic voltage variations, flicker, an injection of currents with non-sinusoidal waveforms (i.e. harmonics), and the like.
These effects can be undesirable for end-user equipment and other generators or components connected to the grid, such as transformers. As the power capacity increases, an evident need arises for improving the power quality characteristics of the turbine output. The power quality impact of a wind turbine depends on the technology involved with it. Despite this fact, wind turbines manufacturers did not consider the power quality as a main design feature.
Originally, the first wind turbines were designed to work at a fixed rotational speed. According to this model, the wind turbine's generator is directly connected to the grid and operates at a determined speed, allowing very minor speed variations. In the case of an asynchronous generator, only the slip range of the generator is allowed. The slip being the difference in the rotation speed of the rotor as compared to the rotating magnetic field of the stator. The generator's slip varies slightly with the amount of generated power, and it is therefore not entirely constant. Furthermore, these wind turbines need starting current limitation strategies and reactive energy compensation elements during normal operation. Wind turbulence produces a non-desirable torque variation which is directly transmitted to the wind turbine's drive train and, hence, to the active power fed to the electrical grid.
A type of wind turbine that keeps the rotational generator speed proportional to the wind speed, is a variable speed wind turbine. In order to obtain the maximum efficiency of the wind turbine, the generator rotational speed adapts to the fluctuating wind speed. This type of wind turbine includes power electronic converters that are connected to the grid. Due to this kind of interface, harmonic emissions from the turbine's power electronic converters are fed into the grid.
Presently wind turbines of the variable speed type using power electronic converters have become widespread. Examples of this variable speed wind turbine are described in U.S. Pat. No. 5,083,039, U.S. Pat. No. 5,225,712 or U.S. Published Application 2005/0012339. These turbines, based on a full converter system, include a generator, a converter on the generator side, a DC link Bus, and an active converter connected to the grid. The variable frequency energy of the generator is transferred to the DC link Bus by the generator side converter, and later transformed to a fixed frequency by the grid side active converter. Some disadvantages are common to all full converter systems. The active switching of the semiconductors of the grid side converter injects undesirable high frequency harmonics to the grid. To avoid the problems caused by these harmonics, a number of filters must be installed. Furthermore, due to the different impedance values on the grid and previously existing harmonics, different tuning of the filters is required according to the characteristics of the wind farm location.
Another variable speed wind turbine is described in the U.S. Pat. No. 6,137,187. As shown in FIG. 1, this wind turbine configuration includes a doubly fed induction generator (1), a power converter (4) comprising an active converter on the rotor side (5), a DC Bus (8), and an active converter on the grid side (7). In this configuration, only a minor part of the total power is transferred through the converters (5, 7) to the grid (9). Power can be delivered to the grid (9) directly by the stator (3), whilst the rotor (2) can absorb or supply power to the grid (9) via the power converter (4) depending on whether the doubly fed induction generator is in subsynchronous or supersynchronous operation. Variable speed operation of the rotor has the advantage that many of the faster power variations are not transmitted to the network but are smoothed by the flywheel action of the rotor. However, the use of power electronic converters (4) connected to the grid (9) causes harmonic distortion of the network voltage.
Other documents also describe variable speed wind turbines. For example, U.S. Pat. No. 6,933,625 describes a variable speed system which includes a doubly fed induction generator, a passive grid side rectifier with scalar power control and dependent pitch control. In this case, there is an active converter on the rotor side, a passive grid side rectifier and a switchable power dissipating element connected on the DC link Bus. During supersynchronous operation the energy extracted from the rotor is dissipated in the switchable power dissipating element, reducing the efficiency of the wind turbine; during the operation of the wind turbine in the subsynchronous mode, the energy is rectified by the passive grid side rectifier which causes undesirable low frequency harmonics in the grid. Thus, complex attenuation filters are required. U.S. Pat. No. 6,566,764 and U.S. Pat. No. 6,856,038 describe variable speed wind turbines having a matrix converter. Both cases include power electronic converters connected to the grid, which may cause undesired harmonic voltages.
All the previously mentioned U.S. Patents and other existing solutions on variable speed wind turbines that include power electronics have converters connected to the grid. Depending on the technology used on the converters, there are different ranges of harmonics introduced on the grid which must be attenuated by using filters, and tuned to the final application location, making the systems more expensive and less reliable.
In view of these problems in the prior art, there is a need to provide an improved power solution, which may be applied to variable speed wind turbines.
Another undesirable problem, especially in the case of weak grids, is the reactive power consumption during the synchronization of the generator. For example, a synchronization method is described in the U.S. Pat. No. 6,600,240. This method starts connecting the generator stator to the grid while the power converter is disabled and the rotor has reached a predefined speed. At this moment, the full magnetizing current is supplied by the grid, which causes a reactive power consumption. This reactive power consumption is sometimes not allowed by some new grid compliance regulations. This patent also describes a disconnection process. The process starts reducing the rotor current and disabling the rotor converter. In this moment, the reactive magnetizing current is supplied by the grid. To disconnect the generator the contactor is opened with reactive current, decreasing the operational life of the contactor. Accordingly, there is a need to provide a method for synchronization, connection and disconnection to the grid of the doubly fed induction generator, which avoids the consumption of reactive power and increases the lifetime of connecting devices.
Another aspect that determines the power quality injected to the grid is the control of the generator. One type of control of the generator side converter is known as “field orientated control” (FOC). The FOC method is based on the electrical model and the parameters of the machine. Due to the dispersion of the machine parameters, the torque can not be accurately calculated, and additional online adjusting loops are required. Moreover, the FOC method that is used introduces delays in the flux position identification when a fault occurs in the grid, making it more difficult to fulfill the new grid compliance requirements.
In prior art variable speed wind turbines with DFIG configuration, although the stator power remains constant, the rotor power is also fed into the grid through the power converter. Due to the rotor power ripple, the total power fed into the grid is also rippled, affecting the output power quality of the wind turbine.
Variable speed wind turbines, which only use a doubly fed induction generator, cannot use electric braking. As described above, in this kind of configuration, power is delivered to the grid directly by the stator, and a minor part of the total power is transferred from the rotor to the grid through the converters. When an incidental stop of the wind turbine occurs, for example during a persistent fault in the grid, the generator's power decreases sharply. Only fast non-electrical braking, such as blade pitching, can be applied to stop the wind turbine. This operation mode produces great mechanical strengths in wind turbine components, which may cause premature damages. Thus, there exists a need for additional braking to prevent this mechanical stress.
The use of high voltage DC link transmission (HVDC) in wind farms is described in Patent No. WO01/25628, which includes a synchronous generator as the main generation device. Due to the use of synchronous machines, the output frequency varies with the wind, so especially at low wind conditions, the ripple content of the output DC voltage becomes high. Furthermore, the output transformer and rectifier must be oversized because they must be able to operate at low frequency. Additional details, such as special construction of the rotor circuitry with low inductance, are mandatory for the accurate regulation of the output power.